Liquid injected helical screw compressors are characterized by a liquid circulation system wherein a liquid such as mineral oil is continuously injected into the compression chamber for cooling, sealing and lubrication functions. The liquid is discharged with the gas being compressed by the compressor, is separated from the compressed gas, cooled, and recirculated back into the compression chamber. Circulation of the liquid may be provided by the differential pressure between the liquid reservoir and the compression chamber at the point of injection of liquid or a pump driven by a power takeoff shaft may be provided to circulate the liquid. Liquid flow into the compressor must occur early and at adequate rates during compressor startup to prevent overheating and failure of the compressor rotor bearings and other working parts requiring lubrication and cooling. Multistage compressors are particularly difficult to start without overheating due to the fact that compression starts immediately in the first stage but liquid reservoir pressure does not increase fast enough to assure adequate liquid circulation due to the throttling effect or flow resistance caused by the second or subsequent compressor stages. This condition is further aggravated in a compressor wherein the first stage compression ratio is greater than the second stage compression ratio since a relatively rapid and greater temperature rise occurs in the first stage compression chamber and relatively greater bearing loadings are experienced.